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How Does a Buyer Work by Using AQL?

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RedSheep Trading uses the mature method of AQL and this stands for: “Acceptance Quality Limit”. We use AQL to perform a product quality inspection. However, what exactly is that? What makes this the standard? In this article, we’ll take you by the hand and offer you an in-depth understanding of the standard of quality control.

Acceptance Quality Limit

The standard definition of Acceptance Quality Limit (AQL) indicates the utmost allowable percentage of defective goods (or the number of maximum defects per one hundred pieces) on a batch of products, which can be thought of as a satisfactory result for the sample.

AQL pic

Every production process has a defective rate, even if just a slight one. No industry will 100% be warranted that goods can be free from errors. Therefore, we can only think about the large recalls of the various automotive company that received so much interest in the media. The AQL is a statistical method of quality determination.

How the AQL value is decided


The identification letter

First, the entire amount of all produced products – the batch size (first column in table 1) – is recorded. The inspection level should then be determined. A distinction is made between general and special inspection levels. Every level is once more divided into different subclasses (columns in Table 1). As a result of these determinations (the intersection in table 1), the identifier (from A to R) is then obtained.

Table one: Identification letters for the sample size based on DIN ISO 2859 part 1

An example:

A purchaser determines his batch size at 30,000 pieces. He then selects the inspection level per the standards applicable to his production, e.g. the general inspection level II. The intersection between batch size and inspection level yields the identification letter – in this case, M.

The AQL value

The size of the partial amount to be inspected – the sample – can be checked in another table (table 2) based on the determined identification letter (the vertical plane in table 2). Based on the previous example, here’s a sample of 315 pcs.

Table 2: Simple sampling for normal inspection (according to DIN ISO 2859 part 1)

In the next step, the acceptable quality level – the AQL value – is decided (the horizontal plane in Table 2). This can be determined by legal standards in most areas. If an AQL value of 2.5 is fixed for the previously mentioned example, the numbers 14 and 15 are read at the intersection of the identifier M and AQL = 2.5. This concerns the acceptance and rejection amount. In this example, the batch is accepted for 14 defective goods found, rejected for 15 defectives, since the total amount doesn’t meet the prescribed quality criteria.

In the case of 15 defective goods, the buyer should reject the batch and ask the defective goods to be sorted, to be repaired, or to request replacement goods. The buyer should then have his batch tested once again.

On the other hand, if less than 15 defect products are found for these 315 inspected goods, this stands for that there’s a 95% odds of finding less than 5% (15/315) defective goods in the whole batch.

The most often used values ​​by purchasers are the following AQL values: 2.5 / 4 / 6.5 for critical/major/minor defects.

The confidence level

If just a fixed partial amount is checked from a total amount, no 100% accurate statement can be made from this regarding the quality of the total amount. However, precise conclusions regarding the level of quality could be drawn from statistical methods and empirical experience. The statistical probability that the result of the sample has validity for the total quantity is referred to because the confidence level and could be determined using adequate calculations and tables. The result is a percentage.

RedSheep Trading could offer higher AQL rate quality inspection service

Thanks to our Changshu office, which is 30 mins distance to most of your shopfitting equipment factories, we can offer a higher AQL rate quality inspection service. If you need, we can offer a critical standard, even a 1 by 1 product quality inspection, by using this service, we help you reduce defect rate to 0, which means you won’t get any bad reviews from customers.

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